This is typically done with three or more people. When a guy cums after having sex, he will typically ejaculate into one person's mouth. After he is done, the person with all the semen will give some to the other person and typically make out with each other so the semen is exchanged.

Male: I'm gonna cum!
Female 1: Cum in my mouth for me! Mmmm, it tastes so good!
Female 2: I want some too! Why not do a sperm swap?